He may have violated Greyhound policy, but I don't see a UUW.
If the guy is on the up & up, his recourse is a 42USC sect. 1983 lawsuit. I hope he has the resources and support. As a company policy Greyhounds only recourse is to not allow you to ride. It would not even arise to requiring a call to the police unless they have asked you to leave and you refused. This is a perfect illustration not to let fear prevent you from otherwise enjoying a lawful activity. If they want to harass you they will charge you with anything, proving it is another story.
It would be nice to know what specific AGG UUW provision was cited in the arrest?